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Diffstat (limited to 'g10/seskey.c')
| -rw-r--r-- | g10/seskey.c | 359 |
1 files changed, 359 insertions, 0 deletions
diff --git a/g10/seskey.c b/g10/seskey.c new file mode 100644 index 0000000..1549017 --- /dev/null +++ b/g10/seskey.c @@ -0,0 +1,359 @@ +/* seskey.c - make session keys etc. + * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, + * 2006, 2009, 2010 Free Software Foundation, Inc. + * + * This file is part of GnuPG. + * + * GnuPG is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 3 of the License, or + * (at your option) any later version. + * + * GnuPG is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program; if not, see <https://www.gnu.org/licenses/>. + */ + +#include <config.h> +#include <stdio.h> +#include <stdlib.h> +#include <string.h> + +#include "gpg.h" +#include "../common/util.h" +#include "options.h" +#include "main.h" +#include "../common/i18n.h" + + +/* Generate a new session key in *DEK that is appropriate for the + * algorithm DEK->ALGO (i.e., ensure that the key is not weak). + * + * This function overwrites DEK->KEYLEN, DEK->KEY. The rest of the + * fields are left as is. */ +void +make_session_key( DEK *dek ) +{ + gcry_cipher_hd_t chd; + int i, rc; + + dek->keylen = openpgp_cipher_get_algo_keylen (dek->algo); + + if (openpgp_cipher_open (&chd, dek->algo, GCRY_CIPHER_MODE_CFB, + (GCRY_CIPHER_SECURE + | (dek->algo >= 100 ? + 0 : GCRY_CIPHER_ENABLE_SYNC))) ) + BUG(); + gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM ); + for (i=0; i < 16; i++ ) + { + rc = gcry_cipher_setkey (chd, dek->key, dek->keylen); + if (!rc) + { + gcry_cipher_close (chd); + return; + } + if (gpg_err_code (rc) != GPG_ERR_WEAK_KEY) + BUG(); + log_info(_("weak key created - retrying\n") ); + /* Renew the session key until we get a non-weak key. */ + gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM); + } + log_fatal (_("cannot avoid weak key for symmetric cipher; " + "tried %d times!\n"), i); +} + + +/* Encode the session key stored in DEK as an MPI in preparation to + * encrypt it with the public key algorithm OPENPGP_PK_ALGO with a key + * whose length (the size of the public key) is NBITS. + * + * On success, returns an MPI, which the caller must free using + * gcry_mpi_release(). */ +gcry_mpi_t +encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits) +{ + size_t nframe = (nbits+7) / 8; + byte *p; + byte *frame; + int i,n; + u16 csum; + gcry_mpi_t a; + + if (DBG_CRYPTO) + log_debug ("encode_session_key: encoding %d byte DEK", dek->keylen); + + csum = 0; + for (p = dek->key, i=0; i < dek->keylen; i++) + csum += *p++; + + /* Shortcut for ECDH. It's padding is minimal to simply make the + output be a multiple of 8 bytes. */ + if (openpgp_pk_algo == PUBKEY_ALGO_ECDH) + { + /* Pad to 8 byte granulatiry; the padding byte is the number of + * padded bytes. + * + * A DEK(k bytes) CSUM(2 bytes) 0x 0x 0x 0x ... 0x + * +---- x times ---+ + */ + nframe = (( 1 + dek->keylen + 2 /* The value so far is always odd. */ + + 7 ) & (~7)); + + /* alg+key+csum fit and the size is congruent to 8. */ + log_assert (!(nframe%8) && nframe > 1 + dek->keylen + 2 ); + + frame = xmalloc_secure (nframe); + n = 0; + frame[n++] = dek->algo; + memcpy (frame+n, dek->key, dek->keylen); + n += dek->keylen; + frame[n++] = csum >> 8; + frame[n++] = csum; + i = nframe - n; /* Number of padded bytes. */ + memset (frame+n, i, i); /* Use it as the value of each padded byte. */ + log_assert (n+i == nframe); + + if (DBG_CRYPTO) + log_debug ("encode_session_key: " + "[%d] %02x %02x %02x ... %02x %02x %02x\n", + (int) nframe, frame[0], frame[1], frame[2], + frame[nframe-3], frame[nframe-2], frame[nframe-1]); + + if (gcry_mpi_scan (&a, GCRYMPI_FMT_USG, frame, nframe, &nframe)) + BUG(); + xfree(frame); + return a; + } + + /* The current limitation is that we can only use a session key + * whose length is a multiple of BITS_PER_MPI_LIMB + * I think we can live with that. + */ + if (dek->keylen + 7 > nframe || !nframe) + log_bug ("can't encode a %d bit key in a %d bits frame\n", + dek->keylen*8, nbits ); + + /* We encode the session key according to PKCS#1 v1.5 (see section + * 13.1.1 of RFC 4880): + * + * 0 2 RND(i bytes) 0 A DEK(k bytes) CSUM(2 bytes) + * + * (But how can we store the leading 0 - the external representaion + * of MPIs doesn't allow leading zeroes =:-) + * + * RND are (at least 1) non-zero random bytes. + * A is the cipher algorithm + * DEK is the encryption key (session key) length k depends on the + * cipher algorithm (20 is used with blowfish160). + * CSUM is the 16 bit checksum over the DEK + */ + + frame = xmalloc_secure( nframe ); + n = 0; + frame[n++] = 0; + frame[n++] = 2; + /* The number of random bytes are the number of otherwise unused + bytes. See diagram above. */ + i = nframe - 6 - dek->keylen; + log_assert( i > 0 ); + p = gcry_random_bytes_secure (i, GCRY_STRONG_RANDOM); + /* Replace zero bytes by new values. */ + for (;;) + { + int j, k; + byte *pp; + + /* Count the zero bytes. */ + for (j=k=0; j < i; j++ ) + if (!p[j]) + k++; + if (!k) + break; /* Okay: no zero bytes. */ + k += k/128 + 3; /* Better get some more. */ + pp = gcry_random_bytes_secure (k, GCRY_STRONG_RANDOM); + for (j=0; j < i && k ;) + { + if (!p[j]) + p[j] = pp[--k]; + if (p[j]) + j++; + } + xfree (pp); + } + memcpy (frame+n, p, i); + xfree (p); + n += i; + frame[n++] = 0; + frame[n++] = dek->algo; + memcpy (frame+n, dek->key, dek->keylen ); + n += dek->keylen; + frame[n++] = csum >>8; + frame[n++] = csum; + log_assert (n == nframe); + if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe)) + BUG(); + xfree (frame); + return a; +} + + +static gcry_mpi_t +do_encode_md( gcry_md_hd_t md, int algo, size_t len, unsigned nbits, + const byte *asn, size_t asnlen ) +{ + size_t nframe = (nbits+7) / 8; + byte *frame; + int i,n; + gcry_mpi_t a; + + if (len + asnlen + 4 > nframe) + { + log_error ("can't encode a %d bit MD into a %d bits frame, algo=%d\n", + (int)(len*8), (int)nbits, algo); + return NULL; + } + + /* We encode the MD in this way: + * + * 0 1 PAD(n bytes) 0 ASN(asnlen bytes) MD(len bytes) + * + * PAD consists of FF bytes. + */ + frame = gcry_md_is_secure (md)? xmalloc_secure (nframe) : xmalloc (nframe); + n = 0; + frame[n++] = 0; + frame[n++] = 1; /* block type */ + i = nframe - len - asnlen -3 ; + log_assert( i > 1 ); + memset( frame+n, 0xff, i ); n += i; + frame[n++] = 0; + memcpy( frame+n, asn, asnlen ); n += asnlen; + memcpy( frame+n, gcry_md_read (md, algo), len ); n += len; + log_assert( n == nframe ); + + if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe )) + BUG(); + xfree(frame); + + /* Note that PGP before version 2.3 encoded the MD as: + * + * 0 1 MD(16 bytes) 0 PAD(n bytes) 1 + * + * The MD is always 16 bytes here because it's always MD5. We do + * not support pre-v2.3 signatures, but I'm including this comment + * so the information is easily found in the future. + */ + + return a; +} + + +/**************** + * Encode a message digest into an MPI. + * If it's for a DSA signature, make sure that the hash is large + * enough to fill up q. If the hash is too big, take the leftmost + * bits. + */ +gcry_mpi_t +encode_md_value (PKT_public_key *pk, gcry_md_hd_t md, int hash_algo) +{ + gcry_mpi_t frame; + size_t mdlen; + + log_assert (hash_algo); + log_assert (pk); + + if (pk->pubkey_algo == PUBKEY_ALGO_EDDSA) + { + /* EdDSA signs data of arbitrary length. Thus no special + treatment is required. */ + frame = gcry_mpi_set_opaque_copy (NULL, gcry_md_read (md, hash_algo), + 8*gcry_md_get_algo_dlen (hash_algo)); + } + else if (pk->pubkey_algo == PUBKEY_ALGO_DSA + || pk->pubkey_algo == PUBKEY_ALGO_ECDSA) + { + /* It's a DSA signature, so find out the size of q. */ + + size_t qbits = gcry_mpi_get_nbits (pk->pkey[1]); + + /* pkey[1] is Q for ECDSA, which is an uncompressed point, + i.e. 04 <x> <y> */ + if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA) + qbits = ecdsa_qbits_from_Q (qbits); + + /* Make sure it is a multiple of 8 bits. */ + if ((qbits%8)) + { + log_error(_("DSA requires the hash length to be a" + " multiple of 8 bits\n")); + return NULL; + } + + /* Don't allow any q smaller than 160 bits. This might need a + revisit as the DSA2 design firms up, but for now, we don't + want someone to issue signatures from a key with a 16-bit q + or something like that, which would look correct but allow + trivial forgeries. Yes, I know this rules out using MD5 with + DSA. ;) */ + if (qbits < 160) + { + log_error (_("%s key %s uses an unsafe (%zu bit) hash\n"), + openpgp_pk_algo_name (pk->pubkey_algo), + keystr_from_pk (pk), qbits); + return NULL; + } + + + /* ECDSA 521 is special has it is larger than the largest hash + we have (SHA-512). Thus we change the size for further + processing to 512. */ + if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA && qbits > 512) + qbits = 512; + + /* Check if we're too short. Too long is safe as we'll + automatically left-truncate. */ + mdlen = gcry_md_get_algo_dlen (hash_algo); + if (mdlen < qbits/8) + { + log_error (_("%s key %s requires a %zu bit or larger hash " + "(hash is %s)\n"), + openpgp_pk_algo_name (pk->pubkey_algo), + keystr_from_pk (pk), qbits, + gcry_md_algo_name (hash_algo)); + return NULL; + } + + /* Note that we do the truncation by passing QBITS/8 as length to + mpi_scan. */ + if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG, + gcry_md_read (md, hash_algo), qbits/8, NULL)) + BUG(); + } + else + { + gpg_error_t rc; + byte *asn; + size_t asnlen; + + rc = gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, NULL, &asnlen); + if (rc) + log_fatal ("can't get OID of digest algorithm %d: %s\n", + hash_algo, gpg_strerror (rc)); + asn = xtrymalloc (asnlen); + if (!asn) + return NULL; + if ( gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, asn, &asnlen) ) + BUG(); + frame = do_encode_md (md, hash_algo, gcry_md_get_algo_dlen (hash_algo), + gcry_mpi_get_nbits (pk->pkey[0]), asn, asnlen); + xfree (asn); + } + + return frame; +} |